1. Field of the Invention
The present invention relates to an electron microscopy employing a CCD detector in order to acquire a backscattered electron image, a transmission electron image, or a scanning transmission electron image.
2. Description of the Related Art
When an electron beam enters a specimen in an electron microscopy, a part of the electron beam is scattered at a large angle by an atom in the specimen. Then, the scattered electron beam discharges electrons (backscattered electrons) from the surface of the specimen while losing little energy. Another part of the electron beam discharges electrons (transmission electrons) behind the specimen, without being absorbed in the specimen.
The backscattered electrons can be mainly classified into two types: high angle backscattered electrons scattered at high angles from the surface of the specimen and low angle backscattered electrons scattered at low angles from the surface of the specimen. The high angle backscattered electrons mainly have information on composition distribution, while the low angle backscattered electrons mainly have information on depression and projection. By discriminating these two types of backscattered electrons from each other, higher composition contrast and higher depression and projection contrast can be obtained.
For example, elastically scattered electrons having energy equal to that of the incident electron beam are mainly scattered at high angles, have information on the vicinity of the surface of the specimen, and include little information on the inside of the specimen. Therefore, a surface composition image of comparatively high resolution can be obtained by discriminating and detecting only the electrons scattering at high angles.
Various methods are used as the method for such discrimination, including: a method of colliding backscattered electrons with an electrode for low angle and an electrode for high angle, respectively, converting the backscattered electrons into secondary electrons, and detecting the secondary electrons; a method of selectively detecting high angle backscattered electrons and low angle backscattered electrons by use of an aperture; and a method of emphasizing stereoscopic effects by dividing a detecting face of a backscattered electron detector into two to five, and calculating a signal between each of the divided detecting faces.
On the other hand, the information on the inside of the specimen is reflected in a scanning transmission electron image (STEM image). The information in the STEM image is classified into a bright field image and a dark field image in accordance with a scattering angle of the transmission electron. A bright field image signal includes information on density of the specimen and information on electron diffraction. In the case of the dark field image, different information is visualized depending on a detected angle. Since a heavier element causes the transmission electrons to scatter at larger scattering angles, a difference of the atomic number of the specimen can be visualized. For this, discrimination of the bright field from the dark field is important. Discrimination of the bright field from the dark field has been conventionally performed using a selected area aperture.
For example, WO2000/19482 has disclosed a method of converting low angle backscattered electrons into secondary electrons, and detecting the secondary electrons while attaining a short working distance. Japanese Patent Application Publication No. 2002-110079 has disclosed a method of adding low angle backscattered electrons, high angle backscattered electrons, which are discriminated from each other and detected, and the secondary electrons at any selected ratio. Japanese Patent Application Publication No. Hei 11-273608 has disclosed a method of selectively detecting high angle backscattered electrons and low angle backscattered electrons using an aperture. Additionally, Japanese Patent No. 3776887 has disclosed a method of changing a range of the scattering angles of the transmission electrons to be detected, by making a position of a transmission electron detector variable.